Not sure if I'm reading this wrong when you refer to working out the curvature measurement, but generally one would mount a piece a wood between centers on the lathe and with a cutting tool held in place by a tool post you would then turn down the wood into a cylinder of whatever the desired diameter was measured with calipers.

Ferg, are you implying that the curvature on the desired block is other than circular, e.g., elliptical instead? (With major axis = 41 mm and minor axis = 40 mm? Seems like that would be an awfully round ellipse though.)

I'm pretty sure that uilleann sanding blocks are usually circular in cross section, with diameter being the only important metric.